Sale of weapons like S-300 missiles would make it harder for the US and other countries to intervene militarily.

Russia has defended its sales of anti-aircraft systems to the regime of Syrian President Bashar Assad, just days after joining forces with the US for a new push to end Syria's civil war through negotiations.

Russian Foreign Minister Sergey Lavrov avoided saying whether those sales included advanced S-300 batteries. Israel has asked Russia to cancel what it said was the imminent sale of the S-300 missiles, portrayed by US Secretary of State John Kerry as destabilising to Israel's security.

The S-300s would make it harder for the US and other countries to even consider intervening militarily or enforcing a no-fly zone in Syria. The US has urged Russia - an Assad ally along with China, Iran and the Lebanese Hezbollah militia - to cut off weapons supplies to Syria.